Keywords: soil-structure-interaction, finite-infinite-element modelling, soil half-space, non-linear structural behaviour, water tank, cooling tower, monitoring system.

In order to activate structural reserves to the ultimate load capacity for large concrete shell structures like cooling towers, a close-to-reality description is required, which considers the typical characteristics of all components - structure and soil, and their reciprocal effects. Computation models, which modify the structures with idealised boundary conditions, like spare masses, springs and dampers, are usually not sufficient and not adequate for the advanced structural model. More comprehensive analysis models are needed, considering both the geometrical and material non-linearity of the concrete structure and its interaction with the supporting soil as realistic as possible.

A finite-infinite-element formulation [1] permits holistic modelling of the structure and soil and its reciprocal static and dynamic interaction. The soil region (half-space) will be subdivided into a near and a far field. The near field underneath the foundation will be modelled by means of the finite element method and may consider material non-linearities. The far field will be modelled by means of an infinite element method (finite elements for unbounded domains) and is assumed to behave linearly.

The shape function of an infinite element can be characterised by two dominant shape functions: a displacement shape function decreasing in the infinite direction and a geometric shape function increasing in the infinite direction. Whereas the deformation of an infinite element in the infinite direction reaches zero, the geometric position vectors in this direction reach infinity. The three-dimensional infinite element considered here possesses only one infinite direction. The geometric shape function results from multiplication of the shape function of the two-dimensional finite direction in phi,eta with the geometric shape function of the one-dimensional infinite direction in xi according to [2]. The displacement shape function will be composed likewise. These shape functions are valid both for Cartesian and axis-symmetric convective coordinate systems and thus will fit perfectly to convective curvilinear shell formulations.

In the paper more details are presented and benchmark with verification examples are given for an elastic block under an impulsive loading, a liquid storage tank under earthquake excitation and a cooling tower founded on inhomogeneous inclined half-space.

Furthermore a research and development project is shown, from which significant results for the formulation of holistic structural and geotechnical models are to be expected. Besides the verification of soil models, the extensive monitoring sensor-equipment in and underneath a large-scale power plant foundation enables a deeper knowledge about the settlement behaviour and its effects on the structure to be received. From these results, features to prevent structural damage may be deduced, but as well conclusions for an economic design of future power plants and of foundations for other large-scale and sensitive buildings can be drawn.

1

E. Mahran, "Finite-Infinite-Elemente-Modellierung zur Simulation der Boden-Bauwerk-Flüssigkeit-Interaktion unter dynamischer Anregung", Bergische Universität Wuppertal, Institut für Konstruktiven Ingenieurbau, Heft 4, 2004.

2

N.A. El-Esnawy, A.Y. Akl, A.S Bazaraa, "A new parametric infinite domain element", Finite Elements in Analysis and Design, 19, 103-114, 1995. doi:10.1016/0168-874X(94)00060-S

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